In general, wireless communications systems using beamforming make use of a number of transmit and/or receive antennas and signal processing to create fixed or adaptive transmit/receive beam patterns. The beam patterns may have a directional nature that may result in a performance improvement when compared with unidirectional transmit and/or receive antennas. The use of the beam patterns may yield a transmit/receive gain over wireless communications systems using unidirectional transmit and/or receive antennas.
As such, beamforming has been promoted as a promising technique to increase cell coverage and to improve cell edge spectral efficiencies. However, one main drawback of beamforming is the so called flashlight effect where the channel quality changes between user equipment (UE) measurement and reporting, and NodeB (NB) transmission, due to the changes in the beam pattern of the interfering neighbouring cells. Coordinated beamforming/switching has been suggested as a possible solution to this problem (see C80216m-08—487, Alcatel_Lucent, “Grid-of-Beams (GoB) Based Downlink Multi-User MIMO”, IEEE802.16m, May 2008; NTT DoCoMo, “Proposals for LTE-Advanced technologies”, R1-082575, Warsaw, Poland, Jun. 30-Jul. 4, 2008; and co-assigned U.S. patent application Ser. No. 12/412,624, filed Mar. 27, 2009, entitled “System and Method for Wireless Communications,” which are incorporated herein by reference).
During the development of Long Term Evolution (LTE), there was much discussion for the flexibility to customize LTE to maximize performance in specific environments. Also, during discussions of LTE-Advanced (LTE-A), many suggestions were made to manage interference level by using coordinated transmissions from different cells (see Ericsson, RI-082469, “LTE-Advanced-Coordinated Multipoint transmission/reception”, Warsaw, Poland, Jun. 30-Jul. 4, 2008; and Huawei, R1-083236, “Coordinated Beamforming for Interference management in Advanced E-UTRA”, Jeju, Korea, Aug. 25-29, 2008, which are incorporated herein by reference). It is well known that as cell size is decreased in an effort to improve spectral efficiency, interference increases.
What is needed, then, is a method of coordinated beamforming where the coordination information between the cells is very limited and typically changes slowly, and where the coordination requires no additional signaling on the radio interface.